1. Field of the Invention
The present invention is directed to a method of forming paper wherein the furnish is supplied to a dewatering press, and thereafter, to a repulping mixer prior to being supplied to a headbox of a papermaking machine.
2. Description of Background Art
Tissue products can be surprisingly difficult to form. It is necessary for the tissue be both soft and strong and also possess good formation or uniformity. The tissue product must be produced at very low cost implying that the process must be carried out at very high speed. Typically, tissue is formed by depositing a very thin layer of a very uniform dispersion of fiber in a carrier on a support which moves at high speed, the dispersion being referred to as the furnish and the support as the wire. Usually, the furnish is a two phase furnish of fiber dispersed in a continuous phase of water. Recently, processes employing three phase furnishes have been developed using an aqueous foam as the carrier for the fiber. However, to disperse the fiber in the foam with the required degree of uniformity and then maintain that degree of uniformity until the furnish can be deposited on the wire can be quite difficult. Further, many types of difficult-to-disperse fibers are known which could be advantageously incorporated into tissues but for the difficulty involved in dispersing them.
One approach is set forth in the Eber et al Patent, U.S. Pat. No. 4,488,932, which is commonly assigned herewith, wherein a large inventory of fiber-containing foamed furnish is maintained in a mix tank at a mix tank consistency of between about 0.3 to about 4.0% fiber by weight, preferably between 1.5 to 4.0%. An agitator provides the requisite energy to disperse the fibers rapidly, but gently such that wetting of the treated fibers is minimized. The foamed furnish of treated and untreated fibers leaves the mix tank through a line to a twin screw pump which provides the motive energy therefor. Residence time is quite low in the mix tank, typically below 5 minutes, preferably below 3 minutes, for greater retention of high bulk properties of the treated fibers. Retention of the treated fiber characteristics is furthered by the utilization of foamed liquid as the dispersing media, the bubbles in the foamed liquid apparently adhering to and forming a film on the surface of the fibers, particularly the treated fibers, thereby decreasing the potential for fiber wetting even in the presence of mild agitation.
Care is required in the design of agitator members disclosed in the Eber et al Patent. The agitator members are adapted to provide good dispersion of the fibers. Recommended agitation members are low shear agitators with multiple level axial flow impellers in a baffled tank. Variable speed agitation drives are desirable to allow adjustment to minimum mixing energy required for blending the fiber dispersion and operation at energy levels such that turbulence is minimized, yet is sufficient to adequately disperse the fibers. Turbulence is also minimized by proper design of the mix tank.
In operation, procedures using the configuration described in the Eber et al Patent have been found to provide insufficient control over basis weight of the web leading to excessive variability in product properties.
To circumvent the problems involved in producing commercially acceptable tissue using the disclosure set forth in the Eber et al Patent, a procedure described in commonly assigned pending U.S. application Ser. No. 07/599,149, filed Oct. 17, 1990, in the name of Dwiggins et al, was developed to control the basis weight and formation. In this procedure, a controlled feed of fiber at a consistency of 0.5 to 7 wt% is introduced directly to an inlet of the fan pump and is thus mixed with foam to form a furnish having the desired consistency. The procedure of Dwiggins et al has been found to provide control over basis weight and formation. However, an increase in the amount of foam which must be treated in the surfactant recovery systems is produced in the procedure disclosed by Dwiggins et al.